Electrically controllable optical filter device

ABSTRACT

An electrically controllable filter device is provided which comprises an electrode structure which induces a filter for waves within a given wavelength range. The electrode structure is so formed that it within the given wavelength range acts as a filter merely upon electrical feeding whereas it in the absence of electrical feeding has no filtering effect. Furthermore, the electrode structure is so arranged that the filter depending on how the electrode structure is fed can be tuned to a number of different discrete frequencies.

BACKGROUND

The present invention relates to an electrically controllable filterdevice. Such devices find their application within optics, but alsowithin microwave technology etc. In the case of optical filters inwaveguide form these are mostly fabricated in such a way that thegrating is obtained in a boundary surface between two dielectrics closeto the waveguide, e.g. through etching. Another way to obtain a filterstructure is to make an electrode structure form a grating. It is inmany cases desirable to be able to tune the grating as well as tocontrol its strength which means that the effective refractive index ofthe waveguide mode has to be varied etc.

Devices as referred to above are known. These devices, however, work insuch a way that a grating is introduced through the mere existence ofthe electrodes. The grating effect thereof is in some cases evenstronger than that which can be controlled via electrical feeding. Aconsequence hereof is that the grating cannot be "shut off". This meansconsequently also that the strength cannot be controlled.

Through the European patent application EP-A-0 267 667 a DFB-laser isknown which comprises an etched grating. With the use of a so calleddivided electrode structure the current injection can be controlled sothat the period can be modified and it is thus possible to have aninfluence on those parts of the grating which are to be thepredominanting. A grating is, however, always present and it cannot beshut off. U.S. Pat. No. 4,008,947 shows an electro-optic switch or amodulator wherein a grating can be created through the application of avoltage. However, this grating is not tunable and a change inperiodicity cannot be achieved. So called DFB-, and DBR- lasers normallyuse an optical filter structure in a waveguide form wherein the gratingfor example is etched in a boundary surface between two dielectricsclose to the waveguide as mentioned above. The grating thereby acts aswhen the grating period is λ/2. If it, however, is desired to tune thegrating a change in effective refractive index for the waveguide mode isrequired which gives rise to a number of problems. It also involvessubstantial difficulties to control the strength of the grating.However, a strength control of the grating could be achieved through theuse of a periodical variation in refractive index (real and/or imaginarypart) induced in an electrical way through a periodical electrodeconfiguration instead of an etched grating. The refractive index maye.g. be influenced through the electrooptic effect or through injectionof charge carriers. However, as mentioned above, the electrodes as suchgive rise to a grating effect which particularly is so strong that itexceeds the desired grating effect which can be controlled electricallyand it is impossible to completely eliminate the grating effect or to"shut off" the grating.

SUMMARY

It is an object of the present invention to provide an electricallycontrollable filter device which is tunable as well as it is possible tocontrol the strength of the grating. It should also be possible to "shutoff" the grating, e.g. to render it inactive so that the grating effectessentially is eliminated. A further object of the invention is toprovide a device which is comparatively simple and cheap to fabricate.Furthermore, it is an object of the invention to provide a device with agreat flexibility and which can be varied in a number of different ways.A further object of the invention is to provide a device which can beused for optical waves as well as for other waves such as e.g.microwaves. Those as well as other objects are achieved through deviceas initially stated which comprises an electrode structure inducing afilter for waves within a given wavelength range and which is so formedthat it, within the given wavelength range, acts as filter merely uponelectrical feeding whereas it in the absence of electrical feeding doesnot act as a filter and in that, the electrode structure is so arrangedthat the filter, depending on the electrode structure and the feedingthereof, can be tuned to a number of different discrete frequencies.

In one advantageous embodiment the device is so formed that it acts as aBragg-reflector. According to a further embodiment the device is active,e.g. an active optical filter. Still another embodiment relates to anapplication in the form of a directional coupler.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described referring tothe accompanying drawings in an explanatory and by no means limitingway, wherein

FIG. 1 schematically illustrates a filter device comprising a waveguideand an electrode element,

FIG. 2a schematically illustrates a device according to FIG. 1 in anelectrooptical application forming a passive filter,

FIG. 2b is a plan view of the device of FIG. 2a,

FIG. 3a schematically illustrates a device forming an active filter orlaser in cross-section,

FIG. 3b is a longitudinal view of the device of FIG. 3a,

FIG. 4 schematically illustrates an embodiment comprising two waveguidesand forming a grating assisted directional coupler,

FIG. 4a is a simplified cross-sectional view of FIG. 4,

FIG. 4b schematically illustrates an electrode structure-couplingarranged in groups of a grating assisted directional coupler,

FIG. 5 shows an alternative embodiment of a grating assisted directionalcoupler,

FIG. 5a schematically illustrates an example of feeding of the device ofFIG. 5,

FIG. 6a schematically illustrates an embodiment of a divided electrodestructure with alternate feeding of consecutive electrode elements,

FIG. 6b shows an embodiment of a divided structure with alternativelyfed electrode structure,

FIG. 6c illustrates a further embodiment of feeding of a dividedelectrode structure,

FIG. 7 schematically illustrates an example of varying electrodeperiodicity.

DETAILED DESCRIPTION

FIG. 1 shows a simplified embodiment of an electrically controllablefilter device 10 comprising a waveguide with an electrode configurationwhich is so formed that it comprises an electrode structure whichcomprises an electrode element C comprising a first electrode A and asecond electrode B respectively, wherein the electrodes A and Brespectively comprise a number of electrode fingers a₁, a₂, . . . andb₁, b₂ . . . respectively wherein the fingers are arranged in aninterdigital or interlocking manner. The electrode structure may alsocomprise several electrode elements or a great number. In thisembodiment C designates one or more electrode elements for reasons ofsimplicity. If the electrodes A, B have the same potential, or in theabsence of any feeding, a passive grating period Λ₀ is obtained with aperiod of approximately λ_(B) /4, λ_(B) being the so called Braggwavelength. A grating with this period essentially has no influence onthe propagation of waves, i.e. the grating is rendered inactive or shutoff. If, however, different voltages are applied to the electrodes A andB respectively, a grating is obtained which has an effective gratingperiod Λ_(eff) which approximately is equal to λ_(B) /2, wherein λ_(B)represents the so called Bragg wavelength. In this case a strongreflection is obtained and the strength of this can be controlledelectrically from the starting value (0) and upwards. Consequently, noreflection is obtained without application of a voltage whereas if avoltage is applied, Bragg reflection is obtained. In FIGS. 2a and 2b adevice is more thoroughly illustrated which in this case iselectrooptically constructed i.e. based on the electrooptical effectthrough an electrooptical substrate. In FIG. 2a an electrode element Cis illustrated which comprises a first electrode A and a secondelectrode B, wherein the first and the second electrodes A, Brespectively comprise a number of electrode fingers a₁, a₂, . . . andb₁, b₂, . . . respectively wherein the electrodes A, B are connected toa voltage source V₀ and wherein the circuit comprises a contact breaker.FIG. 2a illustrates the electrode structure with an underlying opticalwaveguide 2 and a substrate 1 e.g. of LiNbO₃. Of course also othermaterials can be used. In FIG. 2b a transversal cross-section of thedevice is shown wherein a substrate 1 (e.g. LiNbO₃) with a firstrefractive index n₁ in which runs an optical waveguide 2 with arefractive index n₂. An electrode structure in the form of an electrodeelement C comprising metal, is arranged on top. In this particularembodiment, forming a passive filter, the refractive index n₁ can beapproximately the same as n₂ and for example be about 2,2. Thedifference between the retractive indices, n₂ -n₁ may be approximatelye.g. 10⁻² or in some cases somewhat more. The waveguide 2 may have athickness of approximately 0,5-2 μm and a width which is about 5 μm, butthese values are merely given as examples. The length of the device canbe of the cm order of magnitude. The optical waveguide 2 has arefractive index n₂ which somewhat exceeds the refractive index n₁ ofthe substrate 1 and may for example be fabricated through Ti Indiffusion or proton exchange of the LiNbO₃ -substrate in a way which isknown per se. The device functions in such a way that if the contactbreaker (FIG. 2a) is disconnected, a grating is present which has theperiod Λ₀ as described above and which thus does not have an influenceon lightwaves with the wavelength 4·Λ₀. With a connected contactbreaker, a grating is induced which has the periodicity Λ_(eff) andwhich has a strong influence on a wave with a wavelength λ approximatelyequal to 2·λ_(eff). Depending on how the electrodes are fed the filtercan be tuned to a number of different, discrete frequencies. As anexample, with the free wavelength Λ₀ approximately equal to 1,5 μm, λgets approximately equal to 1 μm and Λ_(eff) approximately gets equal to0,5 μm.

In FIG. 3 is shown an example of the filter device in a charge carrierinjection application forming an active filter or laser. For a filterdevice 20 of this type the electrode structure can be essentially thesame as the electrode structure of the in the foregoing illustratedembodiment whereas the difference lies in the optical waveguide 2' whichnow has an electronic function in a manner known per se. FIG. 3a shows afilter device 20 in a transversal cross-section connected to a voltagesource V₀. The device comprises an electrode structure C' which maycomprise one of more electrode elements which will be described lateron. The device furthermore comprises a lower electrode D in a mannerknown per se. In the figure the substrate 1' has a refractive index n₁,which is approximately the same as the refractive index n₂ of anisolating or semi-isolating layer 4' in a manner known per se. Thestructure of the device essentially forms a normal so called buriedheterostructure of a semiconductor laser. It is important that thedistance between the electrode structure C and the active layer 2' iscomparatively small in the shown embodiment. The active layer 2' has arefractive index n₂. which is greater than the refractive index n₃. ofsemi-conducting layer 3', 3' arranged on both sides of the active layer2', which in turn has a refractive index which is greater than therefractive index n₁, of the substrate 1' and the isolating orsemi-isolating layers 4' with refractive index n4 respectively. Theactive layers may particularly have the form of quantum wells or quantumwires. If the active areas or layers are sufficiently thin, quantumwires are obtained. In FIG. 3b the filter device 20 according to FIG. 3ais shown in a longitudinal view wherein the alternating electrodefingers a₁, b₁, a₂, b₂, of the electrodes A, B have an effective gratingperiod Λ_(eff). In the shown embodiment electrodes A (electrode fingersa₁, a₂, . . . ) are fed with a current which is lower than the currentwith which the electrode B (electrodefingers b₁, b₂, . . . ) is fed.Alternatively the electrode A (with thereto belonging fingers) may beleft free. The gaps 4' form isolating gaps which for example maycomprise semi-isolating or semi-conducting material or an oxide whereasthe layers 1', 2', 3' comprise a semiconducting material, wherein theinjection is carried out in the active layer 2'. As an example wouldwith a wavelength λ₀ of about 1,5 μm, λ be approximately 0,5 μm andΛ_(eff) approximately 0,25 μm. The total length of a filter device 20according to what has been described herein can be as much asapproximately 0,1-1 mm. Numerical values are merely given as examples.According to one embodiment of the invention (not further describedhere) may, in addition to a tuning to different discrete frequencies,also a so called continuous tuning be carried out which consists in acontinuous variation in refractive index in a manner known per se.

According to an alternative embodiment the filter device may take theform of a grating assisted directional coupler. In FIG. 4 a filterdevice 30 is shown very schematically which comprises a first and asecond waveguide 7, 8 respectively and a substrate 1'' on top of whichis arranged an electrode structure, here in the form of an electrodeelement C in a manner essentially analogue to what has been describedabove. In FIG. 4a is likewise schematically a transversal view of thedevice 30 shown comprising two different waveguides 7, 8 with refractiveindices n₇, n₈ respectively on which an electrode structure is arrangedand which are arranged in a substrate 1?' with the refractive indexn_(1''). Since the waveguides 7, 8 are different, they have, at the samefrequency, different propagation constants for which k₇ (ω)=2π/λ₇ (ω)and k₈ (ω) =2 π/λ₈ (ω) respectively and the frequency is ω/2π. Thegrating induced through the electrodes A, B, comprised by the electrodestructure which has an effective grating period Λ_(eff) gives rise to acoupling between the waveguides 7, 8 provided that k₇ (ω)-k₈ (ω)=k_(g)wherein k_(g) is the wave number which should be ±2π/Λ_(eff). Analogueto the preceding embodiments the filter device 30 can be controlled viathe electrodes A, B. However, in some cases the electrode periodicity Λ₀should be considerably smaller than the smallest desired gratingperiodicity Λ_(eff) in order to avoid that coupling occurs through thepresence of the electrode structure as such. In order to solve thisproblem the electrodes or the electrode fingers may however be arrangedin groups as schematically illustrated in FIG. 4b and in such a way thatan effective grating period Λ_(eff) is formed between groups ofelectrode fingers.

In FIG. 5 an alternate embodiment of the filter device 30' is shown,each electrode finger forming an electrode which is fed, or controlled,separately.

Herein the (longer) grating period which is required for the couplingbetween the two waveguides is utilised. It is given by the differencebetween the wave propagation constants of the two waveguides andnormally it is about 40-100 times the grating period of a DFB-laser. Theelectrode period may be a fraction of the grating period and since eachelectrode finger is fed or controlled separately, a "sinus-shapedgrating" can be obtained of which the frequency as well as the strengthis tunable.

The distances between possible frequencies are given by the relationshipbetween the electrode period and grating period, i.e. the period whichcouples the waveguide to the average-light frequency which is used. Theelectrode period should, however, not be the same as for a DFB-lasersince coupling between advancing and retrograding wave respectivelyoccurs through the mere presence of the electrode structure.

In FIG. 5a±n, n=0,±1, . . . denote voltage or current level and Λ_(eff)denote the period of the sine-shaped curve. Alternatively the gratingcould e.g. be controlled in such a way that a square wave is obtained.

In the following will be described how the electrode structure can bedivided longitudinally so that it comprises a number of electrodeelements C1, C2, C3, C4, . . . wherein the different electrode elementscan be arranged or fed respectively in different ways so that e.g. afilter device can be obtained which is tunable to a discrete number ofwavelengths. In FIG. 6a is illustrated an embodiment with a number ofelectrode elements C1, C2, . . . each comprising a first and a secondelectrode respectively A₁, B₁ ; A₂, B₂ ; A₃, B₃ ; A₄, B₄ ; . . . whereineach first and second electrode comprise electrode fingers a₁₁, a₁₂,a₁₃, a₁₄ and b₁₁, b₁₂, b₁₃, b₁₄ etc. and analogue for further electrodeelements.

In the embodiment shown in FIG. 6a the first electrodes A₁, A₂, . . .are alternatingly denoted with +, - etc. whereas the correspondingsecond electrodes B₁, B₂, . . . carry the opposite sign. In certaincases the electrode elements are fed in an alternating way with a periodwhich we here call electrodeelement period and which in this particularcase is 2. Plus (+) will in the following mean that an electrode is fedwith a current (or a voltage is applied) which is higher than the onedenoted minus, which thus can be lower than the one denoted plus (+) ormean that no feeding occurs. With polarities as shown in FIG. 6a aneffective grating period is obtained which is approximately Λ_(eff) ×7/8or Λ_(eff) 9/8, i.e. both periods are obtained. Depending on how theelectrodes are fed, different effective grating periods are obtained. Atypical DFB-laser comprises for example more than 1000 grating periodswherethrough a very large number of different variation possibilitiesare obtained; the smallest relative tuning is about 1/the number ofperiods.

In FIG. 6b an alternate embodiment of feeding of electrode elements isshown wherein the electrode elements are fed in the same way in groupsof two, i.e. if two first electrodes in an electrode element are fed inone and the same way they are followed by two first electrodes which arefed differently etc. (Λ_(eff) is approximately 15/16 (and 17/16). Thisis however, merely given as an example. Different groups are possible,i.e. groups could be arranged in various ways, electrodes can be fedseparately or individually etc.

In FIG. 6c a further embodiment is illustrated of how the electrodeelements or the electrodes respectively can be fed. In this embodimenteach electrode element is fed in one and the same way. In the embodimentshown in FIGS. 6a-6c the electrode elements are so arranged in relationto each other that the electrode periodicity is constant all the time.It is, however, also possible to vary the electrode periodicity which isillustrated in FIG. 7. The distance between two electrode fingers a₁₄,b₂₁ belonging to different electrode elements C1; C2 could e.g. here bethe half of the distance Λ₀ /2 between electrode fingers comprised by anelectrode element. In this embodiment the duplicity which has beendescribed in the foregoing (in relation to FIG. 6a) essentially nolonger exists, but the number of wavelengths which can be tuned will belimited.

Through the present invention it is thus possible to tune as well as tocontrol the strength of a filter and it can be said to give rise to anelectrical synthesizing of different spatial frequencies. The inventionmay find its application within a large number of different areas suchas e.g. modulatable filters for large WDM's and for signal processing oras a tunable laser for a WDM.

According to one advantageous embodiment wherein the device is arrangedon a chip a processor could be integrated on the same chip whichcontrols and takes care of numerous connections which are involved.

The invention shall of course not be limited to the shown embodiments,but it can be freely varied in a number of ways within the scope of theclaims. The electrode structure and combinations of electrode elementsmay take many different forms and the feeding thereof can also becarried out in many different ways. Also a number of different materialscan be used and the number of tunable wavelengths can be different etc.

What is claimed is:
 1. An electrically controllable filter devicecomprising:a substrate; an optical waveguide disposed on the substrate;and an electrode structure for inducing a filtering of waves within agiven wavelength range that propagate through the optical waveguide,wherein the electrode structure is so formed with respect to the opticalwaveguide that the electrode structure, within the given wavelengthrange, induces the filtering merely upon electrical feeding of theelectrode structure whereas the electrode structure in the absence ofelectrical feeding does not induce the filtering; the electrodestructure is so arranged that the filter, depending on the electrodestructure and the feeding thereof, can be tuned to a plurality ofdifferent discrete frequencies; the electrode structure in the absenceof feeding has a passive grating period Λ₀, and upon feeding has aneffective grating period, Λ_(eff), which is greater than the passivegrating period Λ₀, and wherein the effective grating period Λ_(eff)through variation in feeding can vary.
 2. A device as in claim 1 whereinthe effective grating period Λ_(eff) is approximately Λ₀ ·2 so that thefilter works as a Bragg-reflector for waves with the wavelengthλ≈2·Λ_(eff).
 3. A device as in claim 1 wherein the passive gratingperiod, Λ₀, approximately takes one of the values Λ_(eff) /4, Λ_(eff)/8, Λ_(eff) /3 and Λ_(eff) /6.
 4. A device according to claim 1, whereinthe electrode structure comprises at least one electrode element,wherein the electrode element comprises at least a first and a secondelectrode.
 5. A device as in claim 4, wherein the first and the secondelectrodes are fed separately and differently.
 6. A device as in claim 4wherein the first and the second electrode have a finger structure,wherein fingers within each electrode are directed towards each otherand at least partially are interdigitally arranged.
 7. A device as inclaim 4 wherein the electrode structure is longitudinally divided in aplurality of consecutive electrode elements, each comprising a first anda second electrode.
 8. A device as in claim 7 wherein all electrodeelements are fed electrically in the same way.
 9. A device as in claim 7wherein the electrode elements comprised of the electrode structure havean alternating polarity so that consecutive first electrodes havedifferent polarities and consecutive second electrodes have differentpolarities.
 10. A device as in claim 7 wherein a polarity of theelectrode elements of the electrode structure is such that two electrodeelements which are equally fed, are followed by two differently fedelectrode elements etc.
 11. A device as in claim 7 wherein an electrodeperiodicity is constant.
 12. A device as in claim 7 wherein one of anelectrode periodicity and an electrode element periodicity varies.
 13. Adevice as in claim 11 wherein the electrode periodicity of a firstelectrode element differs from the electrode periodicity of aconsecutive electrode element.
 14. A device as in claim 5, wherein theelectrode structure induces an optical filter.
 15. A device as in claim14 forming a passive optical filter.
 16. A device as in claim 15,wherein the electrode structure is formed with respect to the opticalwaveguide disposed on an electro-optical substrate.
 17. A device as inclaim 14, wherein the electrode structure induces an active opticalfilter.
 18. A device as in claim 17 comprising one of a DFB-laser and aDBR-laser.
 19. A device as in claim 17 wherein Λ₀ ≈Λ_(eff) /3.
 20. Anelectrically controllable filter comprising:an optical waveguide; and anelectrode structure for inducing a filtering effect for waves within agiven wavelength range that propagate through the optical waveguide;wherein the electrode structure is so formed with respect to the opticalwaveguide that the electrode structure, within the given wavelengthrange, induces the filtering effect merely upon electrical feeding ofthe electrode structure whereas in the absence of electrical feeding thefiltering effect is negligible; the electrode structure is so arrangedthat the filtering effect, depending on the electrode structure and, thefeeding thereof, can be tuned to a plurality of different discretefrequencies; the electrode structure in the absence of feeding has apassive grating period, Λ₀, and the electrode structure upon feeding hasan effective grating period, Λ_(eff), which is greater than the passivegrating period Λ₀, and for a given electrode structure the effectivegrating period Λ_(eff) through variation in feeding can take one of aplurality of different values.
 21. A device according to claim 20wherein Λ_(eff) is approximately Λ₀ ·2 so that the filter works as aBraggreflector for waves having a wavelength λ≈2·Λ_(eff).
 22. A deviceas in claim 20 wherein the passive grating period, Λ₀, approximatelytakes one of the values, Λ_(eff) /4, Λ_(eff) /8, Λ_(eff) /3 and Λ_(eff)/6.
 23. An electrically controllable active optical filter comprising:asubstrate; an active optical layer, disposed on the substrate, forreceiving injected charge carriers; an electrode structure for inducinga filtering effect for optical waves within a given wavelength rangethat propagate through the active optical layer; wherein the electrodestructure is so formed with respect to the active optical layer that theelectrode structure, within the given wavelength range, induces thefiltering effect merely upon electrical feeding of the electrodestructure whereas in the absence of electrical feeding the filteringeffect is negligible; the electrode structure is so arranged that thefiltering effect, depending on the electrode structure and the feedingthereof, can be tuned to a plurality of different discrete frequencies;the electrode structure in the absence of feeding has a passive gratingperiod Λ₀, and upon feeding has an effective grating period, Λ_(eff),which is greater than the passive grating period Λ₀, which variesthrough variation in feeding.
 24. An active optical filter as in claim23 wherein Λ_(eff) is approximately Λ₀ ·2 whereby the filter is aBraggreflector for waves having a wavelength λ≈2·Λ_(eff).
 25. An activeoptical filter as in claim 23 wherein the passive grating period, Λ₀,approximately takes one of the values, Λ_(eff) /4, Λ_(eff) /8, Λ_(eff)/3 and Λ_(eff) /6.
 26. An active optical filter as in claim 23,comprising one of a DFB-laser and a DBR-laser.
 27. A device as in claim1, wherein the electrode structure comprises at least one electrode thatincludes commonly fed electrode fingers, and ##EQU1## (h=1, 2, . . . ,n) is so small that the electrode structure does not cause any couplingin the absence of electrical feeding.
 28. A device as in claim 27wherein that the electrode fingers are connected groupwise so that eachelectrode finger comprises a number of electrode fingers.